The metabolism of proline and pyrroline-5-carboxylate provides a mechanism for the intercompartmental, intercellular and inter-organ transfer of redox potential. Mediated by the transfer of redox potential, pyrroline-5-carboxylate stimulates the pentose phosphate pathway. PP-ribose-P synthesis and nucleotide production. This mechanism links amino acid and nucleotide metabolism. This effect of pyrroline-5-carboxylate has been shown to be synergistic to the effect of growth factors on ribonucleotide synthesis. This effect suggests that pyrroline-5-carboxylate may mediate hormonal effects and, indeed, may act as a "primitive hormone". The concentration of pyrroline-5-carboxylate is especially high in aqueous humor (7-10X plasma) suggesting that the regulatory effects of pyrroline-5-carboxylate may be especially important for ocular tissues. It has also been shown that the synthesis of pyrroline-5-carboxylate from glutamate and its subsequent conversion to ornithine may play a role in maintaining ornithine as a critical intermediate in the urea cycle.